As tires for higher performance are being developed, a rubber having superior fracture properties, good abrasion resistance and low hysteresis loss (low rolling resistance) is desired as the rubber for the tire tread for such tires.
For decreasing the hysteresis loss, cis-1,4-polybutadiene rubber and the like are used. However, the polybutadiene rubber and the like do not have good fracture properties even though they have good abrasion resistance. As a rubber having a remarkably lower hysteresis loss, a solution polymerized butadiene-styrene rubber having the tin-carbon bond in the molecular chain has been developed. However, this rubber does not always have a good balance between wet grip and low rolling resistance.
An elastomeric composition for a tread in which carbon black alone is used as the filler for the rubber described above can realize the desired balance between various properties and performances to some degree. However, simultaneous improvement of wet grip and low rolling resistance to a higher degree has come to be required more urgently in recent years and it is difficult for the rubber described above to satisfy this requirement. In order to solve this problem, there have been disclosed elastomeric compositions in which white carbon such as silica is used alone or in combination with carbon black as the filler. The use of silica in tire tread compositions sometimes improves the balance between wet traction and traction in ice and snow. However, it has been known that diene polymers have low affinity with white carbon such as silica and it is difficult to obtain a homogeneously dispersed rubber composition by milling. To improve this drawback, a large amount of an expensive silane coupling agent is mixed together in many cases. However, since the use of these silane coupling agents in a rubber formulation generates a substantial amount of ethanol in a Banbury mixer, the coupled rubbery mixture may contain undesirable large bubbles that may form blisters or surface defects in the resultant formed articles. Silane coupling agent which is widely used in the rubber industry for this purpose contains free sulfur groups which are undesirable if present in a rubber formulation processed at high mixing temperatures because "scorching" of the rubber may occur. Scorching reduces the processability of the rubber or renders the rubber unusable.
In order to overcome many of these deficiencies the use of an elastomer having a functionality that interacts with silica was proposed. An example of such elastomeric composition is an elastomeric composition containing a polymer modified with an alkoxysilane compound at the end and a silica filler, as displayed in Japanese Patent Publication Showa 52(1977)-5071, Japanese Patent Application Laid-Open Nos. Showa 56(1981)-104906, Showa 62(1987)-50346 and Showa 62(1987)-227908. With these technologies, the physical properties of tires containing the silane modified rubbers, silica and other ingredients could be improved while reducing the necessary amount of the expensive additive Si-69. However, due to the reaction occurring between the silica filler and the hydrocarboxysilanes functionality of the polymers, Mooney viscosities of the resultant compounds were usually too high, thereby creating problems in mixing, in roll milling and in extruding processes. A countermeasure to the high Mooney viscosity was to reduce the molecular weight of the base diene polymer, however this solution created additional problems at the synthetic rubber plant where polymers are made on an industrial scale. These polymers could not be processed by drying to a final rubber bale because of the low molecular weight (Gum Mooney) of the hydrocarboxysilane terminated polymers. Also the raw, low molecular weight diene polymers were susceptible to "cold flow" during storage.